There is an increasing need of delivering wireless technology with broadband capacity for cellular networks. A good broadband system must fulfil certain criteria, such as high data rate and capacity, low cost per bit, good Quality of Service and greater coverage. High Speed Packet Access (HSPA) is an example of a network access technology that enables this.
HSPA is a collection of protocols which improves the performance of existing Universal Mobile Telecommunication Systems (UMTS), which is a third generation (3G) cell phone technology. UMTS uses Wideband Code Division Multiple Access (WCDMA) as air interface for the radio-based communication between user equipment (UE), in form of a mobile terminal, and the base station (BS). The air interface in the Open Systems Interconnection (OSI) model comprises layers 1 and 2 of the mobile communications system, establishing a point-to-point link between the UE and a radio access node (RAN).
WCDMA is a wideband spread-spectrum air interface that utilizes the direct sequence Code Division Multiple Access (CDMA) signaling method to achieve higher speeds and support more users. Key features for WCDMA are:                Two 5 MHz radio channels for Uplink (UL) and Downlink (DL) channels respectively.        Support two basis duplex modes, Frequency division (FDD) and Time division (TDD).        
HSPA is an integral part of WCDMA. Wide-area mobile coverage can be provided with HSPA. It does not need any additional spectrum or carriers. Currently, WCDMA can provide simultaneous voice and data services to users on the same carrier. This also applies to HSPA which means that spectrum can be used efficiently. Simulations show that in a moderately loaded system, HSPA can largely reduce the time it takes to download and to upload large files.
HSPA provides greater system capacity by for instance:                Shared-channel transmission resulting in efficient use of available code and power resources in WCDMA in the downlink (DL).        Fast scheduling prioritizing users with the most favourable channel conditions.        16 Quadrature Amplitude Modulation (QAM) in the DL and the uplink (UL) (as an option 64 QAM for the DL) which results in higher bit-rates.        
The primary benefits of HSPA are improved end-user experience. In practice, this means shorter UL and DL times as a result of higher bit-rates and reduced latency compared to earlier releases of WCDMA. HSPA also benefits operators by reducing the production cost per bit. More users can be served with higher bit-rates at lower production costs.
As with any telecommunication technology, end-user performance with HSPA depends of the type of service and the behavior of higher-layer application protocols. Transmission Control Protocol (TCP) used for packet data services includes slow start and mechanisms which influence the performance, and the overall performance of the service much include these mechanisms. For instance in web-browsing it could be TCP and not HSPA as air interface that limits the performance. In contrast to web-browsing, TCP has very low impact on the time to download a large file, which means the performance is largely determined by the data rate of the radio link. A single user downloading a large file can occupy a significant amount of the total cell capacity.
HSPA is the set of technologies defining the migration path of WCDMA operators worldwide. The two existing features, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), in the HSPA family provides the increased performance by using improved modulation schemes and by refining the protocols by which handsets and base stations communicate. These improvements lead to the better utilization of the existing radio bandwidth provided by UMTS.
High Speed Downlink Packet Access (HSDPA) is the first feature within HSPA. It is part of the WCDMA Third Generation Partnership Project (3GPP) Release 5 specification. HSDPA provides a new downlink transport channel that enhances support for high-performance packet data applications. It represents the first step in the evolution of WCDMA performance. HSDPA can deliver an up to 35 fold increase in downlink data rates of standard WCDMA networks, enabling users to access the Internet on mobile phones and laptops, at speeds previously associated with fixed line DSL.
HSDPA is based on shared channel transmission, which means that some channel codes and the transmission power in a cell are seen as a common resource that is dynamically shared between users in the time and code domains for a more efficient use of available codes and power resources in WCDMA. The radio channel conditions experienced by different downlink communication links vary significantly, both in time and between different positions in the cell.
To compensate for rapidly varying radio conditions in the downlink, HSDPA relies on bit-rate adjustment. That is, while keeping transmission power constant, it adjusts (by lowering) the data rate by adjusting the modulation.
High Speed Uplink Packet Access (HSUPA) is the second feature within HSPA. It is part of the WCDMA Third Generation Partnership Project (3GPP) Release 6 specification. HSUPA provides a new uplink (UL) transport channel called Enhanced Dedicated CHannel (E-DCH). HSUPA dramatically increases the uplink data traffic rate. This technology is likely to significantly increase the amount of data uploaded over mobile networks, especially user-generated content. Although a lot of is downlink oriented, there are still quite a number of applications that will benefit from an improved uplink. These include the sending of large e-mail attachments, pictures, video clips, blogs etc. HSUPA is also known as Enhanced UL.
In contrast to HSDPA, the new uplink channel that is introduced for Enhanced Uplink is not shared between users, but is dedicated to a single user.
FIG. 1 shows a HSUPA network overview. A user terminal UE communicates with the core network CN via at least one base station 11. The system further comprises a second base station 10 with a corresponding system which will be described later. A first radio network controller RNC 12 establishes an E-DCH which enables uplink data traffic from the user terminal to the base station. The E-DCH carries data for at least one radio network bearer. The term “lu” in FIG. 1 represents the interface between RNC and core network. Sometimes the abbreviations lu-ps and lu-cs are used to indicate connection to packet switched or circuit switched core networks. The term “lub” represents the interface between RNC and the radio bases station (RBS).
Several new physical channels are added to provide and support high-speed data transmission for the E-DCH. As shown in FIG. 1, two new code-multiplexed uplink channels are added:                E-DCH Dedicated Physical Data Channel (E-DPDCH)        E-DCH Dedicated Control Channel (E-DPCCH)        
E-DPDCH carries the payload data, and the E-DPCCH carries the control information associated to the E-DPDCH. E-DPDCH is used to carry the E-DCH transport channel. There may be zero, one or several E-DPDCH on each radio link wherein there is at most one E-DPCCH on each radio link.
E-DPDCH and E-DPCCH are always transmitted simultaneously. E-DPCCH shall not be transmitted in a slot unless E-DPDCH is also transmitted in the same slot.
Similarly, three new channels, see FIG. 1, are added to the downlink for control purposes:                E-DCH Hybrid Automatic Repeat Request (HARQ) Indicator Channel (E-HICH) carrying the uplink E-DCH hybrid Acknowledgement (ACK) and Negative ACK (NACK) indicator. The Node B can request retransmissions of erroneously received data packets and will send for each packet either an acknowledgement (ACK) or a negative acknowledgement (NACK) to the UE.        E-DCH Absolute Channel (E-AGCH) carrying absolute grants, which means that it provides an absolute limitation of the maximum amount of uplink resources the UE may use. It is a fixed rate downlink physical channel carrying the uplink E-DCH absolute grant.        E-DCH Relative Grant Channel (E-RGCH) carrying the uplink E-DCH relative grants, which means that it controls the resource limitations by increasing or decreasing the limitations with respect to the current serving grant. The relative grants (RG) are used in the scheduling process to incrementally adjust the allowed UE transmit power. A UE can receive and combine one relative grant from all the E-RGCHs transmitted within the serving radio link set.        
The E-DCH Transmission Time Interval (TTI) can be either 2 ms or 10 ms in length. E-AGCH is only transmitted from the serving cell. E-RGCH and E-HICH are transmitted from radio links that are part of the serving radio link set and from non-serving radio links.
As shown in FIG. 1 the same E-DCH can be provided both through the first RNC 12 for the serving cell and through a second RNC (RNC2) 13 for the non-serving cell. The second RNC 13 serves a separate base station 10 with a Node B NB2 and an enhanced UL scheduler (EUL-S2) (will be described later). Except for E-AGCH (which can only be transmitted through the serving cell) all the physical channels can be transmitted through either of the cells. As an alternative one RNC can serve both a serving cell and a non-serving cell. The term “lur” in FIG. 1 represents the interface between the first RNC 12 and the second RNC 13. Only one RNC will communicate with the core network (i.e. the first RNC). The first RNC is in control of the connection and handles things like soft HO.
The RNC can take the role of serving or drifting. These does not relate to the concept of serving cell or serving radio link RL. The serving RNC is the RNC which acts as the “anchor point” between the radio access network RAN (the radio base station and Node B) and the CN. The serving cell is the best cell in the active set according to some criteria and can belong either to the serving (S-RNC) or the drifting (D-RNC) RNC.
Note that HSUPA channels are added on top of uplink/downlink dedicated channels. Each UE therefore additionally carries an uplink and downlink dedicated physical channel (DPCH), see FIG. 1. In the downlink, a fractional dedicated channel (F-DPCH) can be used alternatively. The F-DPCH carries control information and is a special case of downlink Dedicated Physical Control Channel (DPCCH). UL might only contain the DPCCH as in FIG. 1. It could also contain a Dedicated Physical Data Channel (DPDCH). These have been introduced in 3GPP release 6 in order to optimize the downlink codes usage.
HSUPA scheduling is provided by an enhanced UL scheduler (EUL-S) located in the Node B, see FIG. 1, close to the air interface, but it operates on a request-grant principle where the UE requests a permission to send data and the scheduler decides when and how much data an UE is allowed to send and also how many UEs will be allowed to do so. The EUL-S is located in the Node B in order to move processing closer to air interface and be able to react faster on the radio link situation.
The fast scheduling is used in HSUPA enables rapid resource reallocation between UEs, exploiting the ‘burstiness’ in packet data transmissions. Tasks of the uplink scheduler are to control the uplink resources that the UE in the cell are using. The scheduler therefore grants maximum allowed HSUPA transmission. This effectively limits the transport block size the UE can select and thus the uplink data traffic rate. It enables the system to admit a larger number of high-data rate users and rapidly adapts to interference variations—leading to an increase both in capacity and the likelihood that a user will experience high data rates.
The scheduling mechanism is based on absolute and relative grants. The absolute grants are used to initialize the scheduling process and provide absolute transmit power ratios to the UE, whereas the relative grants are used for incremental up- or downgrades of the allowed transmit power. The absolute grant is carried by the downlink physical channel E-AGCH and the relative grant is carried by the downlink physical channel E-RGCH. The grants are used as a maximum transmission limit on the uplink transmission channel E-DCH. The grants can be converted to the scheduled rate.
Different scheduling strategies can be implemented. This flexibility is useful, as different environments and traffic types can have different requirements on the scheduling strategy. A UE can, for instance, be scheduled from just one base station or from several base stations at the same time.
Macro diversity is exploited for HSUPA, i.e. the uplink data traffic packets can be received by more than one cell. There is one serving cell controlling the serving radio link assigned to the UE. The serving cell is having full control of the scheduling process and provides the absolute grant to the UE. The serving radio link set is a set of cells containing at least the serving cell and possibly additional radio links with common RG generation. The UE can receive and combine one relative grant from the serving radio link set. There can also be additional non-serving radio links. The UE can have zero, one or several non-serving radio links and receive one relative grant from each of them.
In addition to the scheduled mode of transmission (E-AGCH and E-RGCH) the standards also allows a self-initiated transmission mode from the UEs, named non-scheduled. The non-scheduled mode can, for example, be used for Voice IP (VoIP). The UE adjusts the data rate for scheduled and non-scheduled flows independently. The maximum data rate of each non-scheduled flow is configured at Radio Link Setup and/or Radio Link Reconfiguration procedure, and typically not changed frequently.
As a basic principle of the uplink scheduling mechanism, the UE maintains a serving grant which represents the maximum E-DPDCH power ratio the UE may use in the next transmission. The available uplink power determines the possible data rate. The absolute grant allows the Node B scheduler to directly adjust the granted rate of UEs under its control. It is used to initialize the serving grant. The relative grants are used to incrementally adjust the UE's serving grants. As an input to the scheduling, UE feedback is required. The UE has the possibility to send scheduling information which provides detailed information about the buffer status in the UE. Therefore, the Node B scheduler can make appropriate scheduling decisions.
It happens that the UE does not obey its grant and thereby transmit at a too high data rate. This can happen for faulty UE or due to that the UE didn't detect the downlink E-AGCH and E-RGCH physical channels carrying the grant data. Moreover, the UE can always transmit the non-scheduled part according to network configuration and it is only the scheduled part that the Enhanced UL scheduler controls.
This is currently a problem in WCDMA Radio Access Network (RAN). UE sometimes transmit on too high rate, which causes disturbances in the cell. This rate is higher than the rate granted by the scheduler. The scheduler repeating the grant could help, if the UE hear the repeated grant. However, if it is transmitted on the same power level, it might fail again. Just increasing the downlink power every time the E-RGCH or E-AGCH is transmitted is too costly. Still, if the repeating of the grant does not help, something needs to be done with the UE, since the transmitting on a too high rate could seriously disturb the cell.
WO 2006/51867 discloses a mobile communication system in which the UE is instructed to lower the bit rate of E-DCH data channel when the received electrical power of E-DCH is too high. A non-serving cell sends an E-RGCH to instruct the UE to lower the transmission rate of E-DCH when received electric power of E-DCH is high. The problem with this system is that there is no method of handling a situation when the UE does not change the power after the E-RGCH has been sent.